a: 7/15=56/120

-3/8=-45/120

-2/3=-80/120

a: 5/14=15/42

4/21=8/42

b: 4/5=48/60

7/12=35/60

8/15=32/60

a: -5/5=-1=-7/7

8/7=8/7

b: -3/15=-1/5=-6/30

5/6=25/30

c: -34/136=-1/4=-9/36

-12/108=-1/9=-4/36

26/-156=-1/6=-6/36

a: \(\dfrac{11}{42}=\dfrac{11\cdot1}{42\cdot1}=\dfrac{11}{42}\)

\(\dfrac{5}{6}=\dfrac{5\cdot7}{6\cdot7}=\dfrac{35}{42}\)

 b: \(\dfrac{40}{7}=\dfrac{40\cdot4}{7\cdot4}=\dfrac{160}{28}\)

\(\dfrac{10}{28}=\dfrac{10\cdot1}{28\cdot1}=\dfrac{10}{28}\)

c: \(\dfrac{4}{15}=\dfrac{4\cdot4}{15\cdot4}=\dfrac{16}{60}\)

\(\dfrac{14}{60}=\dfrac{14\cdot1}{60\cdot1}=\dfrac{14}{60}\)

a)\(\dfrac{11}{42}=\dfrac{35}{42}\)

b) \(\dfrac{160}{28}=\dfrac{10}{28}\)

c) \(\dfrac{16}{60}=\dfrac{14}{60}\)

`MSC: 756`

Có: `5/21=[5.36]/[21.36]=180/756`

      `3/28=[3.27]/[28.27]=81/756`

      `45/[-108]=[-45.7]/[108.7]=[-315]/756`

Quy đồng \(\dfrac{5}{21};\dfrac{3}{28};\dfrac{45}{-108}\) 

Chuyển dấu \(\dfrac{45}{-108}=\dfrac{45 . \left(-1\right)}{\left(-108\right) . \left(-1\right)}=-\dfrac{45}{108}\) 

Ta có MSC(21,28,108): 756

\(\dfrac{5}{21}=\dfrac{5 . 36}{21 . 36}=\dfrac{180}{756}\) ; \(\dfrac{3}{28}=\dfrac{3 . 27}{28 . 27}=\dfrac{81}{756}\) ; \(-\dfrac{45}{108}=\dfrac{\left(-45\right) . 7}{108 . 7}=-\dfrac{315}{756}\)

a) \(\dfrac{-8}{31}=\dfrac{-8\cdot101}{31\cdot101}=\dfrac{-808}{3131}\)

\(\dfrac{-789}{3131}=\dfrac{-789}{3131}\)

c) \(\dfrac{1}{n}=\dfrac{n+1}{n\left(n+1\right)}\)

\(\dfrac{1}{n+1}=\dfrac{n}{n\left(n+1\right)}\)

a) \(-\dfrac{8}{31}=\dfrac{-8\cdot101}{31\cdot101}=\dfrac{-808}{3131}\)

\(\dfrac{-789}{3131}=\dfrac{-789}{3131}\)

c) \(\dfrac{1}{n}=\dfrac{n+1}{n\left(n+1\right)}\)

\(\dfrac{1}{n+1}=\dfrac{n}{n\left(n+1\right)}\)

a) \(-\dfrac{8}{31}=\dfrac{-8\cdot101}{31\cdot101}=\dfrac{-808}{3131}\)

\(\dfrac{-789}{3131}=\dfrac{-789}{3131}\)

b) \(\dfrac{11}{2^3\cdot3^4\cdot4^5}=\dfrac{11\cdot2^3\cdot5^3}{2^6\cdot3^4\cdot4^5\cdot5^3}=\dfrac{11000}{2^6\cdot3^4\cdot4^5\cdot5^3}\)

\(\dfrac{29}{2^2\cdot2^4\cdot5^3}=\dfrac{29\cdot3^4\cdot4^5}{2^6\cdot3^4\cdot4^5\cdot5^3}=\dfrac{2405376}{2^6\cdot3^4\cdot4^5\cdot5^3}\)

c) \(\dfrac{1}{n}=\dfrac{n+1}{n\left(n+1\right)}\)

\(\dfrac{1}{n+1}=\dfrac{n}{n\left(n+1\right)}\)

Bài 1:

\(\dfrac{-5}{18}=\dfrac{-20}{72};\dfrac{7}{-24}=\dfrac{-21}{72}.\)

\(\dfrac{-15}{-40}=\dfrac{3}{8}=\dfrac{9}{24};\dfrac{24}{-72}=\dfrac{-1}{3}=\dfrac{-8}{24}.\)

Bài 3:

a) \(\dfrac{2}{3}h=\dfrac{8}{12}h;\dfrac{3}{4}h=\dfrac{9}{12}h.\Rightarrow\dfrac{2}{3}h< \dfrac{3}{4}h.\)

b) \(\dfrac{4}{5}km/h=\dfrac{8}{10}km/h;\dfrac{9}{10}km/h.\Rightarrow\dfrac{4}{5}km/h< \dfrac{9}{10}km/h.\)